The present invention relates to an airframe, and more particularly to an airframe structure which attenuates energy during a high sink rate impact event and minimizes the potential that high mass components may enter the crew compartment.
In order to survive high sink rate impacts, helicopters typically utilize a combination of landing gear, fuselage crushing and seat stroking to attenuate energy. While the landing gear is a primary element in attenuating energy in impacts, there are conditions when the landing gear may be rendered ineffective, such as water landings, rough terrain, and when the gear is retracted. In these scenarios, the aircraft fuselage is the primary energy attenuation structure.
The fuselage attenuates energy through various forms of structural deformation which is dependent upon material elongation and strength properties. Failure modes such as buckling, tearing, crippling, stretching, shearing all absorb energy, but these modes may be relatively inefficient, and difficult to predict. One particular complexity with a helicopter is that high mass systems such as engines, transmissions, main rotor systems, etc. are typically located above the crew and passenger compartments. Deceleration forces during an impact event may cause the high mass systems to separate from their mounting points and enter the occupied compartments.
Historical experience shows that conventional fuselage designs have inherent energy absorption capability. This energy absorption capability is exploited in the overall helicopter system crashworthiness design. However, during the aircraft life cycle, aircraft weight typically grows to accommodate updated systems such that he fuselage structure is reinforced to support the extra weight. The reinforced structure typically becomes stiffer which may reduce structural deformation of the fuselage which in turn may induce higher deceleration forces during an impact event. Higher deceleration forces may increase the likelihood that the high mass systems will separate from the mounting points and enter the occupied compartments. The net result may be a decrease in crashworthiness capability of the fuselage.
Accordingly, it is desirable to provide an airframe which attenuates energy during a high sink rate impact event, that is generally independent of structural stiffness and minimizes the potential that high mass components may enter a crew compartment.